Regardless of the performance, you should be able to do this using loc with boolean indexing:

df = pd.DataFrame([[5, 2], [3, 4]], columns=('a', 'b'))

# modify value in column b where a is 3
df.loc[df.a == 3, 'b'] = 6

df
#   a   b
#0  5   2
#1  3   6
Answer from akuiper on Stack Overflow
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Statology
statology.org โ€บ home โ€บ how to replace values in a pandas dataframe (with examples)
How to Replace Values in a Pandas DataFrame (With Examples)
September 27, 2022 - The following code shows how to replace multiple values in an entire pandas DataFrame: #replace 'E' with 'East' and 'W' with 'West' df = df.replace(['E', 'W'],['East', 'West']) #view DataFrame print(df) team division rebounds 0 A East 11 1 A West 8 2 B East 7 3 B East 6 4 B West 6 5 C West 5 6 C East 12 ยท The following code shows how to replace a single value in a single column:
Discussions

python - Replace single value in a pandas dataframe, when index is not known and values in column are unique - Stack Overflow
There is a question on SO with the title "Set value for particular cell in pandas DataFrame", but it assumes the row index is known. How do I change a value, if the row values in the relevant colum... More on stackoverflow.com
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June 20, 2018
python - Trying to change a single value in pandas dataframe - Stack Overflow
I've seen a bunch of questions with similar titles but I still cant figure it out. All I want to do is replace the value thats in my dataframe in the fifth row and 5th column with the value 100. I More on stackoverflow.com
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How to replace a particular Pandas dataframe value by location?
Check out this SO answer, the recommended way is to use pandas.DataFrame.set_value: DataFrame.set_value(index, col, value, takeable=False) Put single value at passed column and index So in your case this would be df.set_value(0, 'dog', 'dog11'). More on reddit.com
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June 30, 2016
python - How do i replace the value of a data-frame column with a single value? - Stack Overflow
Domain expertise still wanted: the latest trends in AI-assisted knowledge for... ... Iโ€™m Jody, the Chief Product and Technology Officer at Stack Overflow. Letโ€™s... 4 Pandas DataFrame, replace value of a column by the value of an other column ยท 0 How to find and replace a single value in ... More on stackoverflow.com
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Python Course
python-course.eu โ€บ numerical-programming โ€บ accessing-and-changing-values-dataframes.php
27. Accessing and Changing values of DataFrames | python-course.eu
So we have to change our DataFrame accodingly: df.replace("programmer", "computer scientist", inplace=True) df ยท to_replace can be a list consisting of str, regex or numeric objects. If both the values of to_replace and value are both lists, they must be the same length.
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datagy
datagy.io โ€บ home โ€บ pandas tutorials โ€บ pandas dataframes โ€บ pandas replace() โ€“ replace values in pandas dataframe
Pandas replace() - Replace Values in Pandas Dataframe โ€ข datagy
March 2, 2023 - The Pandas DataFrame.replace() method can be used to replace a string, values, and even regular expressions (regex) in your DataFrame. The entire post has been rewritten in order to make the content clearer and easier to follow.
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Note.nkmk.me
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pandas: Replace values in DataFrame and Series with replace() | note.nkmk.me
January 17, 2024 - In pandas, the replace() method allows you to replace values in DataFrame and Series. It is also possible to replace parts of strings using regular expressions (regex). pandas.DataFrame.replace โ€” pan ...
Find elsewhere
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Vultr Docs
docs.vultr.com โ€บ python โ€บ third party โ€บ pandas โ€บ dataframe โ€บ replace()
Python Pandas DataFrame replace() - Replace Values
December 27, 2024 - This code snippet substitutes the value 1 in column 'A' with 99. The rest of the DataFrame remains unchanged. Prepare a DataFrame with several integers. Use replace() to swap a list of values with another list.
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Medium
medium.com โ€บ @whyamit101 โ€บ understanding-pandas-replace-function-c2b0b7709233
Understanding pandas replace() Function | by why amit | Medium
February 10, 2025 - Itโ€™s an essential function for data wrangling, especially when your data needs a little tweaking before analysis. Hereโ€™s the basic syntax of the replace() function: ... So, in simple terms, you tell pandas, โ€œFind all instances of to_replace ...
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Medium
medium.com โ€บ @punya8147_26846 โ€บ replace-values-in-pandas-dataframe-9299986d47c3
Replace values in Pandas DataFrame | by Punyakeerthi BL | Medium
June 24, 2024 - pandas, a powerful Python library for data analysis and manipulation, provides the replace() function for efficiently modifying values within a DataFrame or Series.
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Saturn Cloud
saturncloud.io โ€บ blog โ€บ how-to-replace-column-values-in-a-pandas-dataframe
How to Replace Column Values in a Pandas DataFrame | Saturn Cloud Blog
May 1, 2026 - In this example, we created a DataFrame with two columns (โ€˜Aโ€™ and โ€˜Bโ€™). We then used the .replace() method to replace the value โ€˜fooโ€™ in column โ€˜Aโ€™ with the value โ€˜quxโ€™. The inplace=True parameter tells Pandas to modify the ...
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Reddit
reddit.com โ€บ r/learnpython โ€บ how to replace a particular pandas dataframe value by location?
r/learnpython on Reddit: How to replace a particular Pandas dataframe value by location?
June 30, 2016 -

I have the following pandas DataFrame.

import pandas as pd
df = pd.read_csv('filename.csv')

print(df)

     dog      A         B	        C
0     dog1	  0.787575	0.159330	0.053095
1     dog10	  0.770698	0.169487	0.059815
2     dog11	  0.792689	0.152043	0.055268
3     dog12   0.785066	0.160361	0.054573
4     dog13   0.795455	0.150464	0.054081
5     dog14   0.794873	0.150700	0.054426
..    ....
8     dog19	  0.811585	0.140207	0.048208
9     dog2	  0.797202	0.152033	0.050765
10    dog20	  0.801607	0.145137	0.053256
11    dog21	  0.792689	0.152043	0.055268
    ....

I know for row 0 and in column df['dog'], the first value needs to be explicitly changed. Instead of dog1, it should be dog11.

What are all the ways a user can change this value?

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PythonForBeginners.com
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Pandas Replace Value in a Dataframe - PythonForBeginners.com
January 4, 2023 - After execution, the replace() ... to replace different values in different columns with a single final value, you can pass a dictionary to the replace() method as the first input argument....
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Spark By {Examples}
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Pandas Replace Column Value in DataFrame - Spark By {Examples}
November 7, 2024 - In Pandas library there are several ways to replace or update the column value in DataFarame. Changing the column values is required to curate/clean the
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Spark By {Examples}
sparkbyexamples.com โ€บ home โ€บ pandas โ€บ pandas dataframe replace() โ€“ by examples
Pandas DataFrame replace() - by Examples - Spark By {Examples}
October 10, 2024 - pandas.DataFrame.replace() function is used to replace values in columns (one value with another value on all columns). It is a powerful tool for data
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Favtutor
favtutor.com โ€บ articles โ€บ pandas-replace-column-values
Pandas DataFrame: Replace Column Values (with code)
December 15, 2023 - Learn how to replace column values in a Pandas DataFrame using replace, apply and loc methods with Python examples.